This invention relates to radar electronic counter-counter measure systems (ECCM). It is particularly concerned with the defeat of sensitive electronic surveillance measure (ESM) intercept receivers attempting to locate the radar in angle or range, especially while outside the maximum useful detection range of the radar, by comparing measured Doppler shits in the radar's signal.
In connection with this invention "open loop" refers to the fundamental assumption that the ESM system has detected the radar, but the radar has not detected the observer. Since the radar is not tracking the observer attempting passive Doppler location, closed loop countermeasure are not available, where "closed loop" refers to the feedback of target range, speed and heading information to the passive Doppler ECCM system.
The following United States patents are representative of the passive Doppler approaches ESM systems may use, and which this invention will defeat: Shaw et al U.S. Pat. No. 5,241,313 "Angle of Arrival Measurement via Time Doppler Shift" and Tsui et al U.S. Pat. No. 5,315,307 "Doppler Frequency Angle Measurement Technique". These Doppler techniques may be used against pulsed-echo radars.
Techniques exist that confuse a radar receiver attempting to make relative time-of-arrival (TOA) Doppler measurements from received rf pulses. False target signals are generated by receiving the radar signal, varying the relative pulse timing of the signal, and then retransmitting the signal. For example, Barney et al in U.S. Pat No. 3,891,989 "Radar Confusion Countermeasure System" transmit a pulse train identical to those received from the radar in pulse width and frequency, but with random time relationships with respect to the received pulses.
Should it be attempted to use the foregoing techniques for ECCM purposes, ECM techniques such as those described in the Barney patent have severe drawbacks. Although adding pulse groups randomly and thus jittering the pulse repetition interval (PRI) would make estimating pulse time-of-arrival (TOA) difference difficult in the Shaw TOA passive Doppler approach, the pulse repetition interval ("PRI") of an existing radar cannot be randomly altered without changing performance or requiring extensive modifications in processing. To avoid this possible performance degradation and costly equipment change, as well as to avoid possible detection of the counter measure by a pulse deinterleaver in the ESM system, it is desirable to avoid altering the relative position of the pulses in a frame repetition interval (FRI) (FIG. 3) below. Therefore the fundamental pulse repetition frequency, or PRF, must be jittered instead, and this alteration must be done in a manner that does not change PRI processing.
Altering the PRF, however, does not affect an RF frequency Doppler technique such as described in the Tsui patent which could be used in conjunction with the time based technique to check for such countermeasures.
To defeat Tsui's approach the RF carrier frequency must be modulated. Frequency agility is currently used to prevent countermeasures against radar, e.g. such countermeasures as described in the Barney patent. For instance Peters, Jr. et al in U. S Pat. No. 4,068,235 "Frequency Diversity Radar System" describes a method for varying the interpulse frequency of the magnetron in a pulsed oscillator radar.
But as an ECCM technique against passive frequency Doppler measurements, varying interpulse frequency will not work unless the changes in RF frequency are correlated with the PRF changes. Otherwise the presence of frequency modulation can be detected by comparison with a TOA based passive Doppler measurement technique such as Shaw's.